Optical interferometry is a measurement technique that exploits the wave nature of light to produce extremely accurate measurements and provides excellent resolution without requiring any physical contact with the object being examined. Optical interferometry has been used to determine surface textures, shapes, distances, the speed of light through different media, and indices of refraction.
Optical interferometry is based on the phenomenon that two coherent light waves which are brought together (superimpose) behave similarly to water waves rippling through a pond. If the crest of one wave coincides with the crest of anther wave, the waves reinforce one another in what is referred to as constructive interference. If the crest of one wave coincides with the trough of another wave, the waves cancel each other out. This canceling process is referred to as destructive interference. Several wave disturbances arriving at a point simultaneously result in a disturbance that is the vector sum of each of the separate disturbances.
A Michelson interferometer is a well known device that uses interferometry to make extremely precise measurements. In a Michelson interferometer, a light signal is split into two optical beams. The first beam is used as a reference and traverses an optical path of fixed length. The second beam is directed along an optical path which may be varied. The divided beams are recombined to produce an output beam having an interference pattern. The optical path length may be lengthened or shortened to achieve a desired relation between the two divided beams.
One type of Michelson interferometer is described in U.S. Pat. No. 4,190,366. In the system described in the '366 patent, one of the optical path lengths is varied using a wedged-shaped optical element which can be oscillated to provide a correspondingly oscillating optical path length. One limitation of an oscillating wedge-shaped optical element is that the periodicity of the oscillations is limited by the inertial mass of the oscillating apparatus which not only includes the wedged-shaped optical apparatus, but also includes a motor and linkage for driving the optical element. Furthermore, precisely controlling the path length with a mechanical system is difficult with a mechanical apparatus. Such difficulty is especially problematic in applications where good repeatability is desired.
Thus, it may be appreciated that there is a need for an interferometric system in which the optical path lengths of the system may be varied with great precision, good repeatability, and at a frequency greater than that obtainable with mechanically based systems.